The square-planar to flattened-tetrahedral CuX42−(X= Cl, Br) structural phase transition in 1,2,6-trimethylpyridinium salts

Abstract

The abrupt green-to-yellow thermochromism observed for certainA2CuCl4compounds has long been attributed to the presence of strong N—H hydrogen bonding in the low-temperature phase that favors square-planar CuCl42−(green in color) through reduction of ligand–ligand repulsion. Weakening of the hydrogen bonding at higher temperature results in the transformation to (far more common) flattened-tetrahedral geometry – the expected geometry for CuBr42−complexes due to their greater ligand–ligand repulsion. The square-planar to flattened-tetrahedral transitions in (1,2,6-trimethylpyridinium)2CuX4provide the first examples of this phase transformation in the absence of N—H hydrogen bonding and for a CuBr42−complex. These results suggest that the square-planar to flattened-tetrahedral transformation in CuX42−systems may be more common than previously thought.